The distinctive appearance of mother-of-pearl depends on two optical characteristics: pearly luster and iridescence. Pearly or pearlescent luster may be reproduced by the use of nacreous pigments. The earliest such pigment is natural pearl essence, derived from fish scales. More recent nacreous pigments are the metal oxide-coated mica pigments, such as titanium dioxide-coated mica. The second property, iridescence, is a play of colors produced by light interference; certain wavelengths in white light are reinforced while others are suppressed. Certain metal oxide-mica pigments display interference color in addition to pearlescent luster.
Attempts to reproduce the appearance of mother-of-pearl go back many years. Higgins (U.S. Pat. Nos. 1,539,084; 1,606,030; 1,607,623; and 1,607,624) describes the production of pearly celluloid sheets pigmented with fish scale pigment, i.e. natural pearl essence. These sheets have the luster of mother-of-pearl but lack color.
In the U.S. Pat. No. 1,607,622, iridescent color produced from interference by a single thin film of nitrocellulose, polished with a basic halogen salt of bismuth, is combined with the luster of fish scale pigment. The iridescence is very weak, the method is time-consuming and costly, and the product lacks the mechanical strength and wear-resistance for the intended end-uses, e.g. boxes, trays, mirror and brush backs, optical frames and the like.
Interference pigments, for example, titanium dioxide-coated mica in which the titanium dioxide layer is of interference film thickness, may be used in a plastic sheet for both pearl and color effects. However, a given interference pigment is limited by a single reflection color. A combination of different interference pigments incorporated side-by-side in a plastic sheet to increase the color range produces a discontinuous, blotchy effect which lacks the subtlety of the smooth color transitions which are characteristic of natural mother-of-pearl.
A true simulation of mother-of-pearl in a plastic sheet is accomplished in the present invention by combining a pearlescent plastic sheet containing metal oxide-coated mica with an intensely colored multilayer iridescent plastic film. The product has the appearance of natural mother-of-pearl and the mechanical properties necessary for successful use in objects such as compacts, mirror and brush backs, trays, pill boxes, etc.